The current invention is directed to an improved stabilizer for use in a drill string, such as that used to drill a bore through an earthen formation.
In underground drilling, such as gas, oil or geothermal drilling, a bore hole is drilled through a formation in the earth. Bore holes are formed by connecting a drill bit to sections of long pipe so as to form an assembly commonly referred to as a xe2x80x9cdrill stringxe2x80x9d that extends from the surface to the bottom of the bore. The drill bit is rotated so that it advances into the earth, thereby forming the bore. A high pressure drilling fluid, typically referred to as xe2x80x9cdrilling mud,xe2x80x9d is pumped down through the drill string to the drill bit so as to lubricate the drill bit and flushes cuttings from its path. The drilling mud then flows to the surface through the annular passage formed between the drill string and the surface of the bore. The distal end of a drill string, which includes the drill bit, is referred to as the xe2x80x9cbottom hole assembly.xe2x80x9d
A substantial portion of the problems encountered during drilling result from instability in the drill bit and drill string, which places high stress on the drill bit and other components of the drill string. Consequently, drill strings traditionally incorporate one or more stabilizers, which are typically located proximate the drill collar. Such stabilizers typically have pads or blades spaced around their circumference that extend radially outward so as to contact the wall of the bore and, thereby, stabilize the drill string.
Drill bit instability problems are especially prevalent with drill strings employing eccentric drilling elements, such as bi-center drill bits employing a closely coupled pilot drill and a reaming wing, or bottom hole assemblies incorporating a reaming wing separated from the drill bit or without any drill bit. A bottom hole assembly employing an eccentric drilling element can pass through a hole that is smaller than the hole formed by the drilling element. Eccentric drill bits are frequently used to enlarge, or drill an initially large, diameter section of a bore hole that is below a casing having an inside diameter less than that of the hole to be bored. Consequently, conventional stabilizers sized to provide full gauge stabilizationxe2x80x94that is, stabilizers in which the outside diameter of the stabilize is only slightly less than the insider diameter of the bore hole formed by the drill bitxe2x80x94cannot pass through the casing to reach the section to be drilled. As a consequence, full gauge stabilization near the bit cannot be obtained with conventional stabilizers. A lack of full gauge stabilization can result in poor directional control, smaller than expected bore diameter, and excessive stress on the drill bit and drill string.
An improved stabilizer having three axially spaced apart blades, two of which are rotatable, that permits full gauge stabilization of a bi-center drill bit is disclosed in U.S. application Ser. No. 09/532,725, filed Mar. 22, 2000, entitled Drill Bit Stabilizer, hereby incorporated by reference in its entirety. While the stabilizer disclosed in that prior application has many advantages, further improvement in rotatable blade stabilizers, discussed below, are desirable. Consequently, it would be desirable to develop an improved stabilizer that had one or more rotatable blades so as to facilitate stabilization of drill strings employing eccentric drilling elements.
Traditionally, information concerning the properties of the formation being drilled through, such as its density, porosity, electrical resistivity/conductivity, etc., was obtained by a xe2x80x9cwire linexe2x80x9d technique. The technique involved removing the drill string from the bore hole and lowering a device, such as a sonde, which was attached to a cable, into the bore hole. The device typically contained various types of sensors, which may include gamma scintillators, resistivity sensors, nuclear detectors, etc., capable of sensing information concerning the formation. Resistivity sensors, for example, may be used to transmit, and then receive, high frequency wavelength signals (e.g., electromagnetic waves) that travel through the formation surrounding the sensor. By comparing the transmitted and received signals, information can be determined concerning the nature of the formation through which the signal traveled, such as whether it contains water or hydrocarbons. In some applications, the wire line device featured resistivity probes on independently articulated pads mounted on spring actuated arms to ensure that the sensors contacted the bore hole wall.
More recently, xe2x80x9clogging while drillingxe2x80x9d (LWD) systems have been developed in which the sensors are incorporated into the drill string so as to provide real-time information to the drilling operator concerning the properties of the formation being drilled through. In such systems, the information obtained by the sensor is transmitted to the surface, using techniques well known in the art such as mud pulse telemetry, where it is analyzed.
In many types of sensors, it is important that the sensor be as close as possible, and preferably in contact with, the formation so as to minimize errors in the measurement. Consequently, in the past, sensors in LWD systems have sometimes been mounted on the blades of stabilizers used to stabilize the drill string. Unfortunately, such approaches have not been entirely successful in ensuring that the sensor is maintained very close to, or in contact with, the formation. Consequently, it would also be desirable to provide an improved stabilizer that incorporated one or more sensors in such a manner as to ensure that the sensor is maintained very close to, or in contact with, the formation.
It is an object of the current invention to provide an improved apparatus for drilling a bore hole. This and other objects is accomplished in a stabilizer for use in a drill string for drilling a bore hole, comprising (i) a stabilizer body adapted to be mounted in the drill string, (ii) a first stabilizer blade affixed to the stabilizer body, the first stabilizer blade having a distal end adapted to engage the bore hole, (iii) a sleeve rotatably mounted on the stabilizer body, and (iv) at least a pair of circumferentially displaced second stabilizer blades projecting radially outward from the sleeve, the pair of second stabilizer blades rotating in a common circumferentially extending plane axially displaced from the first stabilizer blade.
In a preferred embodiment, the invention further comprises (i) a locking member locking the sleeve into a first position, (ii) means for applying a pressure to the locking member for unlocking the locking member in response to the pressure of the drilling fluid, whereby the sleeve can move into the second position when unlocked, and (iii) means for intensifying the pressure applied to the locking member, whereby the pressure applied to the locking member is greater than the pressure of the drilling fluid. In another preferred embodiment, the stabilizer comprises a sensor for sensing whether or not the stabilizer blade is in a first position. In another preferred embodiment, the stabilizer has a pad mounted in the distal end of the stabilizer blade, the pad having first and second ends, a pivot joint pivotally coupling the pad first end to the blade distal end, whereby rotation of the pad about the pivot joint in a first direction displaces the pad second end radially outward so as to contact the bore hole wall.
The invention also encompasses a method of further drilling a bore hole through an earthen formation, comprising the steps of (i) inserting a drilling string having a stabilizer and an eccentric drilling element into the bore hole, the stabilizer having a rotatable stabilizer blade locked into a first circumferential orientation in which the blade is substantially aligned with the eccentric drilling element, (ii) unlocking the stabilizer blade, and (iii) rotating the unlocked stabilizer blade into a second orientation in which the blade is circumferentially displaced from the eccentric drilling element.
The invention also comprises an apparatus for use in a drill string for sensing a property of an earthen formation through which the drill string drills a bore hole when the drill string is rotated in a first circumferential direction, the bore hole having a wall, comprising (i) a housing adapted to be mounted in a drill string, (ii) a blade extending radially outward from the housing, the blade having a distal end, (iii) a sensor pad mounted in the distal end of the blade, the sensor pad having a circumferentially trailing first end and a circumferentially leading second end when the housing is rotated in the clockwise direction, a pivot joint pivotally coupling the first end of the sensor pad to the blade distal end whereby rotation of the sensor pad about the pivot joint in a counterclockwise direction displaces the sensor pad second end radially outward so as to contact and apply a force to the bore hole wall, the contact creating a friction force when the housing rotates in the clockwise direction that tends to further rotate the sensor pad about the pivot joint in the counterclockwise direction thereby increasing the force applied by the sensor pad second end to the bore hole wall, (iv) at least a first sensor mounted in the sensor pad for sensing a property of the formation.